1. Field of the Invention
This invention relates to a composite of a plurality of yarns having different properties cabled together to form a hybrid cabled cord, and a method to make such a composite. This invention particularly relates to a hybrid cabled cord of polymeric materials having excellent properties at a reduced capital cost and is useful as a tire cord.
2. Brief Description of Related Art
Polyester tire cord has become increasingly common in recent years. Polyester, specifically, polyethylene terephthalate (PET), has many advantages over cords made of rayon or polyamide. Rayon has good dimensional stability but has low strength, poor durability and is hygroscopic. Polyamide has higher strength and durability compared to rayon but has poor dimensional stability. PET tire cord possesses high strength, good durability, better modulus and low creep. Another polyester, polyethylene naphthalate (PEN), provides even better mechanical properties, including modulus, breaking strength and elongation, for use in tires than PET. The use of conventional PEN as reinforcement in the carcass of tires using PEN has been taught, for instance, in U.S. Pat. No. 3,929,180 and U.S. Pat. No. 4,060,516. A PEN yarn having enhanced modulus and dimensional stability (defined as the sum of the elongation at 45 Newton and shrinkage) for use in tires is taught in U.S. Pat. No. 5,397,527. However PEN yarn, both conventional and the improved variety, is notably more expensive than PET, which limits its use in commercial practice to specialty tires such as those needed for high speed performance driving. An ideal tire cord for conventional tires therefore would provide the better mechanical properties of PEN but at the cost of a PET cord. Accordingly, the Inventors have undertaken the search for a solution to making such a tire cord.
It is known to combine yarns having different properties in order to generate a hybrid yarn having improved properties with respect to one or both of the constituent yarns. Examples of hybrid yarns are taught in: U.S. Pat. No. 3,419,059; U.S. Pat. No. 4,155,394; U.S. Pat. No. 5,419,383; U.S. Pat. No. 5,558,144; EP 0335588; and in xe2x80x9cHybrid Tire Cords Containing Keviar Aramidxe2x80x9d by E. R. Barron, Kautschuk+Gummi Kunstoffe 40, Jahrgang, Nr. 2/87, pp. 130-135. Such hybrid yarns typically have an unbalanced construction in that the constituent yarns have unequal twist levels, and, in some instances, there is also a twist level in the cabled cord which is different from the constituent twisted yarns. Typically, the yarn having a higher modulus has a higher twist level compared to the yarn having the lower modulus in order to achieve a cabled cord in which the constituent twisted yarns break simultaneously. Yarns made using this approach show some property improvements, however, yarns of this construction are manufactured using a ring-twister machine. The ring-twister machine is disadvantageous as it requires multiple steps to yield the final product; the constituent yarns must first be twisted separately and then combined in another step to be twisted into the cabled cord. Therefore manufacture cost of such hybrid cabled cords using a ring twister is high.
To reduce or eliminate the cost of using a ring twister in preparing tire cord, U.S. Pat. No. 4,720,943 teaches a cord comprised of a core formed by two to five tire yarns laid side-by-side and wrapped by a wrapper yarn. The wrapper serves merely to hold the core yarns together.
There is prior art suggesting the combination of polyethylene terephthalate and polyethylene naphthalate in cords. In xe2x80x9cPEN Polyester: Potential for Fibers Derived from Naphthalene Dicarboxylate (NDC)xe2x80x9d by Joanna K. Money in Journal of Coated Fabrics, Vol. Jul. 25, 1995 (pp. 24-38) the development of PET/PEN core/sheath fibers is mentioned. Japanese Patent Publication 241026, published Oct. 28, 1991, teaches a hybrid cord formed by twisting or braiding together a plural number of yarns in which the main component is an all aromatic polyester or polyamide yarn with a melting point above 300xc2x0 C. and the other component is a yarn of thermoplastic fiber with a melting point of 110 to 270xc2x0 C. The cord is heat set at a temperature such that the second component softens and deforms to set the cord structure and presumably adds little to the overall mechanical properties of the resultant yarn.
None of the prior art teaches or suggests a hybrid cabled cord comprised of PET and PEN having a coring factor which is not equal to zero and improved tensile strength retention, nor the manufacture of such a cord using a direct cabler.
The present invention is directed to a hybrid cabled cord comprising: at least two yarns having different properties, and an unbalanced configuration and wherein said hybrid cabled cord has improved tensile strength retention. In a preferred embodiment, at least one of the yarns comprises polyethylene naphthalate. In another preferred embodiment, the two yarns comprise polyethylene terephthalate and polyethylene naphthalate and the coring level is between about 3% to about 15%. The present invention is advantageous in providing a cord that is less expensive than a 100% PEN cord yet having comparable properties.
The present invention is also directed to a process for making a hybrid cabled cord comprising the steps of: supplying at least two yarns having different properties to a direct cabler, and cabling the at least two yarns using the direct cabler to achieve a hybrid cabled cord having a coring level which is not zero.